Metered dose inhalers (MDIs) are commonly used to treat a number of medical conditions. Typical medicinal formulations, i.e., solutions or suspensions of drug in a propellant, are housed within a pressurized canister and emitted as an aerosol in controlled amounts by a metering valve acting in conjunction with an actuator. The actuator will generally have a mouthpiece, through which the emitted aerosol may be inhaled by a patient. It is important that the amount of drug reaching a patient with each actuation of the metering valve be a consistent amount, so as to avoid over- or under-dosing of a patient. The form in which the drug is released may also play an important role in providing consistent dosing to a patient. That is, characteristics of the aerosol plume delivered by an MDI, such as the average particle size, the particle size distribution, or the distribution of drug within the particles in the plume, may influence the ultimate effective dose that a patient receives.